1. Field of the Invention
The present invention relates to a method of producing a moisture- and heat-resistant flexible polyurethane foam, and more particularly to a method of producing a moisture- and heat-resistant flexible polyurethane foam, which is free from lowering of the foam hardness due to moisture absorption during or after the production of the foam under a high temperature and high humidity condition and free from development of scorch during the production of the foam.
2. Description of the Prior Art
Flexible polyurethane foam is produced in the form of various products by selecting properly the kind of raw materials and the production methods. Among the production methods, continuous slab foaming process is important for mass production at present. A large amount of cushioning materials for furnitures and beds are produced from the slab stock of flexible polyurethane foam produced by this continuous slab foaming process, through cutting, sewing, riveting, adhesion and the like. In the continuous slab foaming process, a slab stock having a larger dimension is produced more inexpensively, and a flexible polyurethane foam slab stock having a cross-section of about 2 m width and of about 1 m height is produced at present.
Flexible polyurethane foams produced under a high temperature and high humidity condition (particularly, in summer) have a hardness lower than that of foams produced under a dry condition and further are subject to scorch.
However, the mechanism of the above described phenomena have not yet been clarified, and a summer formulation for the reaction in summer has hitherto been empirically set. As the summer formulation, the following formulations have hitherto been used.
(1) A polyol containing a stiffness element incorporated into the polyol itself is used. For example, a blend of a polyol and a polyol grafted with a monomer, such as styrene, acrylonitryl or the like, is used as a polyol component.
(2) Polyurethane foam-forming reaction is carried out at a high isocyanate index of 120-150, which is higher than the commonly used isocyanate index of about 105. (Isocyanate index is a value, which is one hundredfold value of the ratio of the isocyanate group equivalent of an isocyanate compound to the hydroxyl group equivalent of a compound having active hydrogen.)
However, in the method using a blend of a polymer and the polymer polyol, the polymer polyol is a special polyol, and therefore the resulting flexible polyurethane foam is expensive. While, in the method of forming a flexible polyurethane foam at a high isocyanate index, the resulting polyurethane foam is expensive and further scorch is apt to occur. Therefore, there has not yet been found a satisfactory technique for producing a flexible polyurethane foam inexpensively without lowering the foam hardness and without causing scorch under a high temperature and high humidity condition.